EP 1 031 697, in the name of the present Applicant, in particular FIG. 7 of that document, has disclosed a hydraulic actuating device having a throttling assembly which is in communication with the working chamber on the piston rod side.
The throttling assembly is such that when the piston rod is being extended, during the majority of the stroke the fluid can flow out of the working chamber on the piston rod side virtually without being impeded, via a bypass passage. As soon as the piston closes off the opening of the said bypass passage, the fluid can only flow out via a throttling passage, so that the movement of the piston rod is decelerated.
It is explained in EP 1 031 697 that this hydraulic actuating device is advantageous for example in use for driving the roof of a convertible car. In cars of this type, there is generally a windscreen header, against which the front edge of the roof bears in the closed position of the roof, above the windscreen. The design of many roofs is such that in the final section of the closing movement of the roof, the roof as it were drops forwards onto the windscreen header. Without additional measures therefore, the roof would hit the windscreen header with an undesirably hard impact. The known throttling assembly solves this problem by decelerating the final part of the piston rod movement. The switching parameter is in this case the location of the opening of the bypass passage.
One drawback of the abovementioned known actuating device is that an unusual actuator is required, namely an actuator having a bypass passage and associated additional opening which is connected to the cylinder space. As is known, standard double-acting actuators have two connections, each for an associated working chamber.